The invention relates to a connecting element for pipes and a method for welding a connecting element to the end of a pipe. In this connection, the connecting element is designed for being welded to a circular ring-shaped, optionally beveled, end face of a pipe. In particular, the invention relates to a connecting element comprising a conical sealing surface that, when connecting the pipe to other structural components, for example, other pipes, fittings, hoses, and the like, interacts with a sealing surface that is at least partially complementary to it and provided on a connecting element, in particular, a screw fitting or weld fitting listed in DIN EN ISO 8434-1.
Connecting elements of the aforementioned kind are disclosed, for example, in DE 1 960 933 U1. They serve for enabling, together with other structural components, such as, in particular, a screw fitting or weld fitting, a union nut, and an annular seal, extremely stress-resistant seal-tight connections (as they are needed, for example, in high-pressure conduits) of pipes with other structural components, for example, additional pipes, fittings, hoses etc.
The connecting element disclosed in the aforementioned DE 1 960 933 U1 has proven to be extremely successful in practice for many years. However, welding of such a connecting element having a circular ring-shaped end face onto an end face of the same kind provided at the end of a pipe is not trivial and must be performed by qualified, specially trained personnel.
When the usual gas welding method is employed in this connection, as a result of burning of alloy elements contained in the material of the connecting element and of the pipe, which, in the case of the relevant pipelines in this context, are generally both made of steel, cinder is produced on the outer side of the pipe as well as on the inner side of the pipe, wherein the cinder produced on the inner side of the pipe generally must be dissolved by a complex after treatment by etching (conventionally with a 13% hydrochloric acid solution) and subsequently flushed out because otherwise, when using the pipe, for example, in the context of a high-pressure hydraulic oil line, the cinder particles could lead to damages of the structural components, for example, hydraulic cylinders.
In order to prevent cinder formation, usually the so-called tungsten inert gas welding method (TIG welding method) under inert gas atmosphere is employed for welding of the connecting element to the end of the pipe. However, in this connection attention must be paid that during welding neither the material to be welded nor welding beads will reach the interior of the pipe. Subsequently, the welding seam must generally be at least cleaned; frequently, it must also be leveled by grinding.
The known welding methods require not only qualified trained personnel, they are also time-consuming and thus particularly cost-intensive.
Also, weld-free pipe connections in the form of so-called flared fittings are known. In this connection, an end of a pipe is deformed (flared) by a machine and a bead is formed by upsetting, wherein the bead forms a shoulder for a union nut that has been slipped over prior to this. Since the upsetting process requires very great forces and since the pipe must be clamped in a direction counter to the upsetting direction, deep clamping grooves are formed on the exterior of the pipe during upsetting. These clamping grooves can damage the grain within the pipe; in particular, in high-pressure hydraulic systems where the pipelines are frequently subjected to significant dynamic loads, this can cause rupture of the pipe.
Moreover, upsetting produces in the interior of the pipe a channel where dirt can collect; this can lead to increased corrosion in the interior of the pipe.
A problem is also the fact that during flaring the pipe is cold-formed and the course of the grain is not cut; this can also lead to tearing of the grain in the material and thus in rupturing of the pipe occurring more easily.
The flared fittings are therefore suitable only to a limited extent, in particular, for very high operating pressures and in pipeline systems where strong vibrations can occur, for example, in certain hydraulic systems.